The present invention relates to specular-surface bodies which may be used as reflecting mirrors, diffraction gratings and the like as well as in optical devices using mainly high-energy beams, such as X-rays, synchrotron radiation beams (SOR beams), laser beams, etc.; and more specifically, the invention relates to specular-surface bodies in which surfaces of top layers made of silicon carbide are processed to be specular surfaces.
A conventional specular-surface body of the above type, one whose top layer is made of a chemically vapor deposited silicon carbide film (hereinafter called a "conventional specular-surface body") is generally known. That is, the conventional specular-surface body comprises a top layer made of a chemically-vapor-deposited silicon carbide film (hereinafter called a "CVD-SiC film"), with high-purity silicon carbide being chemically vapor deposited on a surface of a substrate made of a silicon carbide sintered material or carbon sintered material. A film surface is then processed to be a super smooth specular surface (surface roughness: RMS 10 .ANG. or less) by a mechanical specular-surface processing method such as polishing, etc.
Conventional mirrors are used as X-ray mirrors, and the like, in which a substrate made of copper, or the like, with gold vapor-deposited thereon, is specular surface polished. Others are used in which a multilayer film, whose film thickness is computed and designed on a basis of a wave length, is coated on a substrate to utilize interference effects. But these conventional mirrors are primarily used for relatively small energy beams with long wavelengths (for example, visible light and infrared rays). High energy beams with short-wavelengths, such as X-rays, etc. are apt to cause peeling-off of such coating layers, distortion of such specular surfaces, heat damage, etc., and it is extremely difficult to deal with these problems.
Compared to this, because the conventional specular-surface body provides excellent optical properties, in that the CVD-SiC film, which is the top layer forming the specular surface, has superior physical properties (such as heat resistance, heat conductivity, fastness, and exhibits a superior optical property of high reflection of beams in the short wavelength region), it does not have the problems described in the preceding paragraph, even when high energy beams in short wavelength regions are used. Thus, it is expected to be preferred as an optical element for a reflecting mirror, diffraction grating, etc. for high energy beams, such as X-rays, and the like, in short wavelength regions.
However, when such conventional specular-surface bodies are irradiated by high energy beams, such as X-rays, etc., an irradiated portion is likely to be damaged, posing the problem that resistance against irradiation by high-energy beams is insufficient.
That is, irradiating the specular-surface bodies with high energy beams, such as X-rays, etc., causes an irradiated portion at a specular surface to have an appearance of fine foam and to look as if the irradiated portion were instantaneously melted; thereby generating visible white turbidity (cloudiness). If this kind of damage occurs on the specular surface, the beam reflectivity inevitably decreases, and it is unable to properly carry out functions required of optical elements, such as reflecting mirrors, diffraction gratings, etc. In addition, an absorption rate of high energy beam increases at the damaged portion, and, in an extreme case, breakage of the specular-surface body itself may result. At the white turbidity portion, silicon is deposited in a form having an appearance of fine liquid drops, indicating that the white turbidity is caused by deposition of silicon.
Therefore, the present inventor conducted various experiments for finding causes of this lack of resistance against irradiation by high-energy beams of the conventional specular-surface bodies (in particular, causes of occurrences of damage due to beam irradiation), and reached the conclusion that the lack of resistance primarily results from absence of a defect-free crystal layer of silicon carbide within a certain depth range in the top layer of the specular-surface body.
That is, specular surfaces in conventional specular-surface bodies, as described above, with surface roughness of RMS 10 .ANG. or lower are obtained by mechanical surface polishing methods (specular-surface processing methods) such as polishing, etc.; but such mechanical surface polishing methods cause, for instance, continuous formation of microcracks. It has been confirmed that a physical impact (hereinafter called a "physical processing force") for scraping away crystals forming irregularities on CVD-SiC film surfaces greatly disturbs atomic arrangements on the specular processed surfaces and portions immediately under the surfaces; thus, layers are formed whose qualities are changed by the machining processing (hereinafter referred to as "quality changed layers") accompanied by a processing strain and/or crystalline dislocation, etc. It has been determined that existence of such quality changed layers result in lowering resistance against beam irradiation and lead to damage from beam irradiation. For example, when high-energy beams are radiated onto a portion where a regularity of atomic arrangement is destroyed, that is, where the energy is high enough to promote rearrangement of atoms, an excessive silicon is deposited outside of a lattice of silicon carbide during the atom rearranging, with the result that white turbidity occurs.
On the other hand, generation of the quality changed layers cannot be avoided by use of an electrical or chemical surface polishing method. Electrical and chemical surface polishing methods do not need physical processing forces, so it is assumed that it is possible to reduce a depth or thickness of the quality changed layer as compared to mechanical surface polishing that requires the physical processing force. Therefore, experiments were carried out using CVD-SiC film coating materials having the defective crystal layers (the quality changed layer) caused by processing strain, etc. which have different depths. the results of the experiments indicated that even if a defective crystal layer is formed in the surface of the CVD-SiC film, no damages due to beam irradiation occurs if the depth of the defective crystal layer is less than a specified level and a defect-free crystal of silicon carbide exists in a range of a specified depth from the film surface. Specifically, even if the quality changed layer or defective crystal layer exists, damages such as formation of the white turbidity does not occur even when high-energy beams such as X-ray, etc. are radiated thereon if the defective crystal layer is extremely shallow and the defect-free crystal layer of silicon carbide exists at a depth within 300 .ANG. from the specular surface.
The present invention was made on the basis of conclusions obtained from findings of these experiments, and it is an object of the present invention to provide a specular-surface body which possesses sufficient resistance against irradiation by high-energy beams, such as X-rays, SOR beams, laser beams, etc. and which can be suitably used as a reflecting mirror, diffraction gratings, etc. for high energy beams.